Why Ext.Ajax.request can't pass by value

Imitate the dynamic generation of grid, of which the main a section of code

var json_f;
Ext.Ajax.request({//The store object needs to develop the fields attribute in the creation, so use ajax first get the column names from the background
url : 'rt.asp', //Read the data returned from the action response, also can get data from other places
method : 'POST',
async : false,
success : function(response) {
//Returns the results are converted to JSON objects, note that the extjs4 function in decode has become: Ext.JSON.decode
s = response.responseText;
json_f = Ext.util.JSON.decode(s); //For backstage pass JSON
}
});